Reflection nebulae occur where the light from a nearby star is reflected off the grains of dust that make up the nebula. The gas in a reflection nebula is cold, and were it not for the light of the neighboring star, these objects would appear as dark nebulae. A good example of a reflection nebula is M78.

Diffuse nebulae are clouds of hydrogen atoms that have been ionized by a nearby star that is massive and hot enough to radiate enough energy to separate the hydrogen into positive hydrogen H+ and free electrons. A good example of a diffuse nebula is the Orion nebula or Messier 42—the brightest diffuse nebula in the sky and visible to the naked eye.

Planetary nebula are the outer layers of stars that are dying but aren't massive enough to go supernova. These are the red giant stars, which our sun may one day become. The name planetary nebula is a bit of a misnomer, as these nebulae are the product of stars, not planets. They were first observed by the French astronomer Charles Messier in 1764, and later named by the discoverer of Uranus, Sir William Herchel, who thought the nebulae appeared similar to Uranus in the telescope. The name stuck, and about 1,600 planetary nebulae have since been discovered, with many more thought to exist throughout the Milky Way. A good example of a planetary nebula is the Ring Nebula or Messier 57.

Supernova remnants are the outer layers of stars that have exploded. The remains are the clouds of dust and gas that once formed the star's outer layers. These clouds are expanding at high speeds—hundreds or even thousands of kilometers per second for recently exploded stars. A good example of a supernova remnant is the Crab Nebula or Messier 1.